It is conventional in the art to pass exhaust gases from diesel engines through a filter to remove particulate (soot or other carbonaceous solids) in the gas before the gas is discharged into the atmosphere. This is generally accomplished by providing a filter, e.g., stainless steel wire mesh, which usually includes a catalyst, the purpose of the catalyst being to facilitate combustion of the particles which are collected on the filter. Such combustion is desirable to avoid filter plugging and the buildup of undesirable back pressure as a result of the collection of soot or like particulate on the filter.
Prior art filter systems have suffered from a number of disadvantages. Thus, for example, with currently available systems, there is a problem of effective combustion of diesel particulate at normal gas temperatures
the temperature of diesel exhaust of 300.degree. C. or below. While the temperature of diesel exhaust gas may be as high as 500.degree. C., it is generally considerably lower, e.g., 300.degree. C. or below, and, as noted, conventional filter arrangements are not particularly effective for combusting particulate at such low temperatures. Means may be provided to increase the temperature of the exhaust gas to above 300.degree. C. to facilitate combustion of the collected particulate but this creates other difficulties. Additionally, prior arrangements have suffered from the substantial generation of sulphuric acid which in turn condenses to form undesired sulphate particulate.
The principal object of the invention is to provide a method for effective combustion and removal of diesel particulate collected on the filter even at low temperatures, e.g., 225.degree.-300.degree. C. Another object is to minimize the formation of sulphuric acid and sulphate particulate. Other objects will also be hereinafter apparent.